1. The Field of the Invention
The invention generally relates to the field of fiber-optic communications. More specifically, the invention relates to the use of optimized avalanche photodiodes in fiber-optic transceivers.
2. The Relevant Technology
In the field of data transmission, one method of efficiently transporting data is through the use of fiber-optics. Digital data is propagated through an optical fiber using light emitting diodes or lasers. Light signals allow for high transmission rates and high bandwidth capabilities. Also, light signals are resistant to electromagnetic interferences that would otherwise interfere with electrical signals. Optical fibers do not typically allow portions of the light signal to escape from the optical fiber as can occur with electrical signals in wire-based systems.
In a typical fiber-optic network, the transmission and reception of data is not strictly limited to optical signals. Digital devices such as computers may communicate using both electronic and optical signals. As a result, optical signals need to be converted to electronic signals and electrical signals need to be converted to optical signals. To convert electronic signals to optical signals for transmission on an optical fiber, a transmitting optical subassembly (TOSA) is often used. A TOSA uses a electronic signal to drive a laser diode or light emitting diode to generate an optical signal. When optical signals are converted to electronic signals, a receiving optical subassembly (ROSA) is used. The ROSA has a photodiode that, in conjunction with other circuitry, converts the optical signals to electronic signals.
Because most computers and other digital devices both transmit and receive signals, most computers need both a TOSA and a ROSA to communicate through optical fibers. A TOSA and ROSA can be combined into an assembly generally referred to as a transceiver. Accordingly, most computers in a fiber-optic network are configured to communicate in a bidirectional nature by using a transceiver, meaning that they can both transmit and receive signals on the fiber-optic network.
Avalanche photodiodes (APDs) are well-known devices that may be used in a ROSA. APDs serve at least two functions: 1) conversion of optical signals into electrical signals, and 2) amplification of the electrical signal through avalanche multiplication. Typically, an APD has an absorption layer where an optical signal is absorbed. The optical signal includes a number of photons. Each photon impinging the absorption layer generates an electron-hole pair or a carrier. A multiplication layer in the APD is designed such that one carrier causes an avalanche of other carriers where the number of other carriers is dependent on the gain of the APD.
Fiber-optic transceivers intended for commercial distribution should be designed and manufactured such that they are functional in a number of different situations. For example, fiber-optic networks may differ in the distance between communication points, the type of fiber between communication points and in other ways. These differences result in differing amounts of signal attenuation in a fiber-optic channel.
One challenge that arises with the use of avalanche photodiodes in commercial transceiver applications is that typically the photodiodes are designed and implemented in a transceiver to achieve maximum gain. In other words, the photodiodes cause the maximum number of carriers to be generated in the multiplication layer when a photon contacts the photodiode. While in long-distance applications high gain is helpful in recovering optical signals, the same photodiodes may cause an overload of current to be delivered to components in a circuit with the photodiode, such as a post amplifier, when the photodiode is used in short distance applications or with optical fiber that exhibit little attenuation.
To effectively provide for this range of currents that may be delivered from the photodiode, namely low currents in long-distance applications and high currents in short distance applications, a post amplifier with a wide dynamic range may need to be implemented. Such amplifiers have various disadvantages including large size, high power consumption and high cost. These disadvantages are directly opposed to the optimal characteristics of transceivers which are compact size, low power use, and low cost.